There is continuing interest in developing electroluminescent devices, such as light emitting diodes, based on organic materials. The primary motivation for this continuing interest is that many organic materials have high fluorescence quantum efficiencies in the visible spectrum, an thus have significant potential for colour display applications. U.S. Pat. No. 5,608,287 describes an example of a typical organic light emitting diode (OLED). Such a device usually comprises a first electrode layer disposed on a substrate, at least one organic layer disposed on the first electrode layer, and a second electrode layer disposed on the organic layer. In operation, a voltage is applied across the organic layer via the electrodes. One of the electrodes (the cathode) injects electrons into the organic layer. The other electrode (the anode) injects holes into the organic layer. Radiative recombination of the oppositely charged carriers produces photon emissions from the device.
The anode is preferably fabricated from a material with a relatively high work function in the interests of providing effective hole injection. Because light has to be transferred out of the device efficiently, it is desirable for at least one of the electrodes to be transparent. In most conventional applications, light is transferred through a transparent anode. Such anodes typically consist of Indium Tin Oxide (ITO) having a work function in the range of 4.2 to 4.8 eV, Zinc Oxide, or Aluminium doped Zinc Oxide. To optimise device performance, the cathode is preferably formed from a material having a relatively low work function preferably aligned to the electron affinity of the organic layer. U.S. Pat. No. 5,677,572 and U.S. Pat. No. 5,776,623 describe multi-layer electrode structures in which different functional layers are stacked to provide a collective effect.